andygrove commented on code in PR #210:
URL: https://github.com/apache/datafusion-site/pull/210#discussion_r4447378615
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content/blog/2026-11-03-datafusion-comet-1.2.0.md:
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+---
+layout: post
+title: Apache DataFusion Comet 1.2.0 Release
+date: 2026-11-03
+author: pmc
+categories: [subprojects]
+---
+
+<!--
+{% comment %}
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+http://www.apache.org/licenses/LICENSE-2.0
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+Unless required by applicable law or agreed to in writing, software
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+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
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+{% endcomment %}
+-->
+
+[TOC]
+
+The Apache DataFusion PMC is pleased to announce version 1.2.0 of the
[Comet](https://datafusion.apache.org/comet/) subproject.
+
+Comet is an accelerator for Apache Spark that translates Spark physical plans
to DataFusion physical plans for
+improved performance and efficiency without requiring any code changes.
+
+This release covers roughly five weeks of development since 1.1.0: 400 commits
from 40 contributors. See the
+[change log] for the full list of changes.
+
+[change log]:
https://github.com/apache/datafusion-comet/blob/branch-1.2/docs/source/changelog/1.2.0.md
+
+Comet has always aimed to return the same results as Spark. Much of this cycle
went into the edge cases that
+remained, working through whole categories at a time (timezones,
floating-point `-0.0` and `NaN`, collations)
+and adding tests that keep them fixed.
+
+The highlights:
+
+- **Results that match Spark**: native timestamp results carry the right
timezone, `-0.0`, `0.0` and `NaN`
+ follow Spark in hashing, comparison, sorting, aggregation and array
functions, and fixes to decimal `AVG`,
+ nested types, broadcasts and join order.
+- **Memory, spill and shuffle at scale**: memory returned to Spark sooner,
fewer memory-related task failures,
+ much smaller JVM shuffle memory use with many partitions, and smaller
shuffle files.
+- **Iceberg**: native writes are a documented, opt-in feature with support for
format-version 3 tables, and the
+ native scan reads fewer nested fields and more files at once.
+- **More of the plan runs natively**: existence joins, `SortAggregateExec`,
the `PIVOT` fast path and
+ HyperLogLog sketch functions, among others.
+- **New places to run**: a native Delta Lake scan (experimental), Arrow Python
UDFs (experimental), and Spark 4.2
+ as a supported version.
+
+## Results That Match Spark
+
+Timestamps and floating point produced the most reports, so we started there,
followed by collations. Individual
+fixes in other areas come after that.
+
+### Timestamps and timezones
+
+Spark labels a timestamp with the session timezone, and Comet's native code
has to produce the same label, not
+only the same instant. Several native expressions got this wrong. 1.2.0 fixes
them:
+
+- Native timestamp results carry the correct timezone label:
`timestamp_seconds` results are labeled as UTC
+ timestamps ([#6341]), `date_trunc` keeps its input's label ([#6345]), and
`CASE` and `COALESCE` relabel
+ timestamp branches instead of panicking ([#6347]).
+- Session timezone IDs are normalized before they reach native code ([#6351]).
+- `date_trunc` with a format that differs from row to row uses the same kernel
as a literal format ([#6354]).
+- The driver warns when the native and JVM timezone databases differ
([#6353]), because Comet cannot correct
+ for that difference.
+- The test harness now fails a test when a native plan mislabels a timestamp,
so this class of bug can't come
+ back unnoticed ([#6868], [#6930]).
+
+Thanks to [@andygrove] for the timezone sweep and [@LinSimon-901101] for the
label checks. The new
+[timezone handling page] in the contributor guide explains the rules.
+
+[timezone handling page]:
https://datafusion.apache.org/comet/contributor-guide/timezones.html
+
+### Floating point: `-0.0`, `0.0` and `NaN`
+
+In some operations Spark treats `-0.0` and `0.0` as equal and every `NaN` as
one value; in others it treats them
+as distinct. Which rule applies depends on the operation and the Spark
version, and DataFusion makes different
+choices. The differences are easy to miss in testing and show up as wrong
joins, groups and sort orders. In
+1.2.0, these follow Spark:
+
+- **Hashing**: `hash`, `xxhash64` and native shuffle hash `NaN` as Spark does
([#6413]).
+- **Comparison and ordering**: `min`, `max`, `greatest` and `least` ([#6457]),
sort and window keys nested in
+ arrays and structs ([#6475]), and `percentile` for negative `NaN`s ([#6829]).
+- **Array functions**: `array_remove`, `sort_array`, `array_distinct` and
`array_union` ([#6518], [#6563]), and
+ `arrays_overlap` on the Spark versions where `-0.0` and `0.0` are one value
([#6989]).
+- **Arithmetic**: `signum(-0.0)` ([#6556]) and the zero check in `Remainder`
([#6896]).
+- **Maps**: Comet no longer builds a map natively when its key type holds a
`FLOAT` or `DOUBLE`, because Spark 4.0+
+ normalizes those keys and rejects duplicates in a way native code did not
([#6992]).
+
+The contributor guide now has a [floating point page] that says which Spark
rule each expression follows and how
+to test for it. A sweep suite crosses edge values with every operator context,
so a DataFusion upgrade that
+shifts float behavior will fail a test before it reaches users.
+
+Thanks to [@andygrove] for the float sweep, [@rich7420] for `percentile` and
[@zhangfengcdt] for `signum`.
+
+[floating point page]:
https://datafusion.apache.org/comet/contributor-guide/floating_point.html
+
+### Collations
+
+Strings with a non-default collation compare differently from ordinary
strings. For collated casts, sort keys and
+comparisons, Comet now falls back to Spark or runs the expression through
[codegen dispatch] instead of comparing
+bytes ([#5302], [#6206], [#6471], [#6480]). Thanks to [@stantheman0128],
[@LinSimon-901101] and [@viirya].
+
+[codegen dispatch]:
https://datafusion.apache.org/blog/2026/06/20/datafusion-comet-0.17.0/
+
+### Other wrong-results fixes
+
+- **Decimal `AVG`**: results were `NULL` or wrong for sums that overflowed an
intermediate `i128`, and empty and
+ overflowing partial states were mixed up. 1.2.0 repairs the division and the
buffers ([#5420], thanks to
+ [@sunchao]) and falls back to Spark for the cases it cannot yet do correctly
([#7017]).
+- **Variance, covariance and correlation**: these now update and merge in
Spark's order, so values that are large
+ and close together give Spark's result, and `regr_r2` returns `NULL` or
`DIVIDE_BY_ZERO` where Spark does
+ ([#6076], thanks to [@rich7420]).
+- **Nested values in comparisons and sorts**: `greatest` and `least` compare
struct arguments by position and
+ order nulls inside nested values first ([#6760], [#6761], thanks to
[@viirya]), and native sort keeps Spark's
+ null ordering inside nested values ([#6968], thanks to [@Weijun-H]).
+- **Missing nested Parquet fields**: when every nested field a query asks for
is missing from a Parquet file,
+ Comet could return a non-null struct, array or map where Spark returns
`NULL`, which changed `IS NULL`, filters
+ and `count`. Comet now follows Spark's version-dependent rules, and falls
back for nested reads of name-mapped
+ Iceberg tables ([#7085], thanks to [@rich7420]).
+- **Nested binary read as strings**: a nested Parquet `BINARY` value read as a
string that held invalid UTF-8
+ became `NULL`. It is now decoded the way Comet's `CAST(binary AS STRING)`
decodes it, replacing ill-formed bytes
+ with U+FFFD ([#7106], thanks to [@manuzhang]).
+- **Sliced broadcasts**: broadcasts of sliced string, binary, array or map
batches could return wrong values or
+ fail with `OversizedAllocationException` ([#6900], thanks to [@comphead]).
+- **Identity broadcasts**: a Spark operator that keeps every row of a Comet
broadcast could see the last row of
+ each batch repeated in place of the others. Comet now copies each row
([#7260], thanks to [@pingzh]).
+- **Join output order**: Spark drops a sort above a hash join because the join
keeps its streamed side's order.
+ Comet's native hash join did not keep it for some inputs, so the query could
return unsorted partitions
+ ([#6835], [#6810], thanks to [@dwsmith1983]).
+- **Spark patch releases**: Spark 4.0.5, 4.1.4 and 4.2.1 change what
`sequence` returns when `stop - start`
+ overflows a long, and the result of casting a zero with a large exponent,
such as `'0e79'`, to decimal. Comet
+ follows the behavior of the Spark version it runs on ([#7171], thanks to
[@snmvaughan]).
+- **ANSI mode**: `array_append` now evaluates its item on rows whose array is
`NULL`, so it raises the errors
+ Spark raises, and `corr` raises `DIVIDE_BY_ZERO` ([#7025], [#6830], thanks
to [@kazuyukitanimura]).
+
+## Memory, Spill and Shuffle
+
+1.1.0 made Comet's memory use visible. 1.2.0 fixes more of the ways it went
wrong, and makes shuffle cheaper.
+
+### Memory
+
+- **Memory is returned sooner.** Native plans return their memory before the
Spark task ends ([#6261]). When
+ Spark has already dropped a task's memory entry, a failed native memory
request or shuffle page allocation is
+ now retried instead of failing the task ([#6310], [#6403], [#6813]).
+- **Spilling works in more places.** A final aggregate that spilled can now
read its spill files back even when
+ they exceed its memory share ([#6544]). A partial merge aggregate whose
input is sorted on some of its grouping
+ keys spills instead of failing the task ([#7222]). Grouped integer `SUM`
state now counts toward the memory
+ reservation ([#6370]).
+- **Container sizing.** The native memory limit follows how YARN and
Kubernetes size executor containers
+ ([#6375]).
+- **Spill files are compressed with lz4 by default.** The codec is set by
+ `spark.comet.exec.spill.compression.codec` ([#6698]). The PR reports TPC-H
q21 going from 380 s to 250 s,
+ against 367 s for Spark.
+- **The memory usage log is in the Spark event log.** It can be read after the
application ends ([#6401],
+ [#6250], [#6271]).
+
+Thanks to [@andygrove] for most of this work, with [@dwsmith1983] and
[@viirya] on the retry fixes and
+[@comphead] on spill compression and the event log.
+
+### Shuffle and broadcast
+
+- **JVM shuffle with many partitions.** The bypass-merge writer gave every
partition a page of Spark's page size,
+ 32 to 64 MB, as soon as it buffered a row. When a task's memory could not
hold one page per partition, every
+ new page forced a spill. Pages now start small and grow ([#7059]). The PR
reports `REPARTITION(64)` on TPC-H
+ `orders` going from 266 s and 2.48 GB to 1.1 s and 156 MB, and on `lineitem`
from 15.5 minutes to 3.3 s.
+- **Smaller shuffle files.** Shuffle files with string, binary, array and map
columns are 9% to 40% smaller in
+ the PR's benchmarks, because offsets are now stored as lengths and
compressed with zstd ([#6943]).
+- **Faster spills.** A JVM shuffle spill writes every partition in one native
call ([#7065]). The PR reports this
+ as 21% to 28% faster with 1,000 partitions, and 45% to 49% faster when the
writer spills every 100,000 rows.
+ Spill pointers are now sorted in the sorter's scratch space ([#7125]).
+- **Shuffle reads.** Shuffle blocks are read in 64 KiB pieces ([#6805]).
+- **Broadcast.** Every task that read a Comet broadcast decoded the whole
broadcast for itself. Tasks on an
+ executor now share one decoded copy ([#7016]). In the PR's benchmark, a
1M-row build side read by 128 tasks on
+ `local[8]` went from 1,321 ms to 684 ms.
+
+Thanks to [@viirya] for the spill pointer sort, [@comphead] for the block
reads and [@andygrove] for the rest.
+
+## Iceberg
+
+### Native writes
+
+Native Iceberg writes stay **off by default** in 1.2.0, as they were in 1.1.0
when they were experimental. We
+considered turning them on, but this is their first release as a documented,
user-facing feature, and we would
+rather users opt in and report what they find first. We plan to revisit the
default in a later release.
+
+To use them, set `spark.comet.write.iceberg.enabled=true`. This replaces the
pair of settings used in 1.1.0, so see
+[Upgrading to 1.2.0](#upgrading-to-120) if you enabled the feature then. What
changed in the writer:
+
+- **Format-version 3 tables**: written natively ([#6672]).
+- **Per-location credentials**: applied to native reads and writes ([#6478]).
+- **Files match iceberg-java more closely**: dictionary encoding is chosen the
way iceberg-java does ([#6241]),
+ partitions past year 262142 match ([#6239]), and float and double identity
partitions are written natively
+ ([#6851]).
+- **Memory**: write buffers are charged to the task's memory pool ([#6247]),
and when the pool refuses a new
+ buffer the writer closes fanout partitions early instead of failing
([#6773]).
+- **Speed**: the writer's dictionary pre-pass is 1.8x to 3.9x faster ([#6937]).
+- **Fallbacks**: more unsupported writes are detected up front and run through
iceberg-java, such as writes with
+ unsupported S3 settings or through a void partition field whose source
column was dropped ([#6441], [#6502],
+ [#6677]).
+
+The [Iceberg guide] lists the supported operations, every fallback, and every
known difference from iceberg-java.
+
+[Iceberg guide]:
https://datafusion.apache.org/comet/user-guide/latest/iceberg.html#writes
+
+### Reads and row-level operations
+
+- The native scan reads only the nested fields Spark keeps ([#6725]) and opens
**two data files at once** by
+ default ([#7132]), which hides the latency of opening files on tables with
many small files or on S3. See
+ [Upgrading to 1.2.0](#upgrading-to-120) for the trade-offs and how to turn
it off.
+- The native scan falls back to Spark when Iceberg reports an output ordering,
since it does not keep that order
+ and Spark would otherwise drop the sort above it ([#7043]).
+- `MERGE INTO` can run natively in `MergeRowsExec` on Spark 3.5+ ([#5318]). It
is off by default; enable it with
+ `spark.comet.exec.mergeRows.enabled`. It also supports `MergeSummary` on
Spark 4.1+ ([#6640]) and Spark 4.2
+ write transactions ([#6587]).
+
+Thanks to [@andygrove] for the writer work, [@snmvaughan] for per-location
credentials, [@unikdahal] for the
+row-level operations, [@comphead] for nested-field pruning, and [@sam-1112],
[@0lai0] and [@parthchandra] for
+fallback gates.
+
+## More of the Plan Runs Natively
+
+- **On by default**: existence joins ([#4587], thanks to [@coderfender]),
queries without a `FROM` clause
+ (`OneRowRelation`, [#6666]), aggregates with neither grouping keys nor
aggregate functions ([#6899]), and `min`
+ and `max` over strings and binary ([#6927]). Broadcast joins whose build
side holds an unsupported scan now run
+ natively too ([#6786], thanks to [@parthchandra]), except that a Text scan
on the build side stays in Spark
+ unless `spark.comet.convert.text.enabled` is set, because converting it
replaces bytes that are not valid UTF-8
+ with U+FFFD ([#7123]).
+- **New operators and expressions**: `SortAggregateExec` ([#4565]), the
`PivotFirst` fast path for `PIVOT`
+ ([#4798]), HyperLogLog sketch functions ([#4802]), distinct-combined
`collect_list` ([#4727]),
+ `AtLeastNNonNulls` ([#6180]), `replace` ([#6680]), hashing of wide decimals
([#6548]), `RANGE` window frames
+ with an offset on a `DATE` key ([#4974]), and interval multiply and divide
([#4715], [#4901]).
+- **Opt-in conversions**: a native `RangeExec` ([#6553]), operators above
typed Dataset operations ([#6564]),
+ native shuffle over row input ([#6607]), and join filter pushdown through
deterministic residuals ([#6431]).
Review Comment:
Good catch, thanks. Added @schenksj and @adivaish as co-authors in the Delta
section, next to @sunchao. My earlier check used the changelog, which drops
co-authors whose commit emails don't map to a GitHub account. So I checked the
Co-authored-by lines on every commit the post references. The only other gap
was @rich7420 on the ANSI `corr` fix (#6830), which is now credited too.
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